1. Field of the Invention
The present invention relates to a shadow-mask color cathode ray tube, particularly to a shadow mask that prevents a beam landing tolerance from decreasing due to the deformation of an electron beam passing hole under press molding.
2. Description of the Prior Art
In general, a color cathode ray tube used for a television receiver or a monitoring terminal comprises a vacuum envelope comprising a panel section for forming an image screen, a neck section for accommodating an electron gun, and a funnel section for connecting the panel section with the neck section; a centering and purity correcting magnetic device externally set to the neck section; and a deflection yoke externally set to the border between the funnel and neck sections.
FIG. 2 is a sectional view of a shadow-mask color cathode ray tube for explanation in which symbol 1 represents a panel section, 2 represents a neck section, 3 represents a funnel section, 4 represents a phosphor layer, 5 represents a shadow mask, 6 represents a mask frame, 7 represents a panel pin, 8 represents a suspension spring, 9 represents a magnetic shield, 10 represents an electron gun, 11 represents a centering and purity correcting magnetic device, 12 represents a deflection yoke, Bc represents a central electron beam, and Bs represents a side electron beam.
In FIG. 2, a phosphor layer 4 is made of a three-color phosphor mosaic formed on the inner surface of the panel section 1, and a shadow mask structure is suspended from the panel pin 7 embedded in the inner wall through the suspension spring 8.
The shadow mask structure comprises the mask frame 6, the shadow mask 5 whose margin is spot-welded to the mask frame 6, and the magnetic shield 9 for shielding the space of the funnel 3 from external magnetism.
The funnel section 3 has the neck section 2 for accommodating the electron gun 10 at its small-diameter end and constitutes a vacuum envelope by frit-welding the open margin of the panel 1 to the large-diameter end margin.
The deflection yoke 12 is externally set to the neck transition portion of the funnel and an image is reproduced by two-dimensionally scanning the phosphor layer 4 formed on the inner surface of the panel section 1 by the electron beam 13 emitted from the electron gun 10.
The centering and purity correcting magnetic device 11 externally set to the neck section 2 is correction means for controlling the hue by adjusting the alignment of the electron-gun and tube axes and adjusting the mutual arrangement between three electron beams.
The shadow mask has the so-called color selecting function for correctly landing three electron beams emitted from an electron gun on a three-color phosphor mosaic constituting the phosphor layer 4 respectively.
The shadow mask is constituted by forming a flat plate into an approximately rectangular semi-finished product having an approximately rectangular effective face area in which a plurality of slot-like electron-beam passing holes are formed in the horizontal and vertical scanning directions of an electron beam and an ineffective area surrounding the effective area, and thereafter forming a skirt section by press-molding the semi-finished product to bend the ineffective area upward at the margin and forming the effective area into an approximately rectangular dome and welding the dome to a mask frame.
FIG. 3a is an illustration of a shadow mask, which is a top view of the shadow mask viewed from the electron gun. FIG. 3b is a cross sectional view of the shadow mask in FIG. 3a, taken along the line X--X of FIG. 3a.
In FIGS. 3a and 3b, symbol 5 represents a shadow mask, 51 represents a boundary (effective border) present at a transition portion between an effective area and a skirt section, 52 represents an effective area in which a slot is formed as an electron-beam passing hole, 53 represents a slot, and 54 represents a shadow-mask developed outline.
The shadow mask is suspended inside the panel section by spot-welding the four corners of the shadow mask to a mask frame (not shown).
FIGS. 4(A) to 4(D) are schematic process diagrams for explaining the outline of a shadow mask manufacturing method in which a shadow mask curved like a dome is formed in the sequence of (A).fwdarw.(B).fwdarw.(C).fwdarw.(D).
First, a number of shadow mask patterns are formed on the low-carbon steel plate 1 shown in FIG. 4(A) by means of photography.
In the case of the shadow mask pattern, a shadow mask unit comprising the effective area 52 in which electron-beam passing holes (slots) are formed and the shadow-mask developed outline 54 having the outer periphery to form a skirt section by bending an ineffective area upward at the margin after press-molding are continuously formed on the low-carbon steel plate 1 and etched to form the slot 53 serving as an electron-beam passing hole.
After annealing, leveling, or surface treatment the shadow mask with the slots 53 is cut along the shadow-mask developed outline 54 to form a semi-finished shadow mask 5' and sent to the press molding process.
In the press molding process (D), the semifinished shadow mask 5' is press-molded by a mold having a domed external form of the shadow mask to obtain the shadow mask 5 shown in FIG. 4(D).
Etched slots formed in the shadow mask for passing the electron beam have their widths increasing or decreasing continuously as they are away from the center. The continuous increase or decrease of the slot width corresponds to the continuous expansion of the electron-beam cross section due to the increase of the deflection angle of an electron bean or the continuous change of the interval between a phosphor layer and a shadow mask tube.
When the shadow mask arranged as described above is press-molded, slots closer to the effective border have larger increase rate of the width or length than those in the central portion of the shadow mask.
That is, because the deformation force applied to a slot formed at the effective border when it is press-molded is larger than that at the central portion, slots located at the effective border, particularly at the corner section have larger increase rates of the width or length than those at the central portion due to the deformation force.
A color cathode ray tube having a shadow mask structure as described has a problem of the so-called decrease of landing tolerance in which the diameter of an electron beam is increased particularly at a corner section and thereby an electron beam is deviated from a predetermined phosphor constituting phosphor mosaic to excite even an adjacent phosphor.
As a result, the color purity is deteriorated and therefore a reproduced image with a high image quality cannot be obtained.
The official gazette of Japanese Patent Laid-Open No. 62436/1991 discloses a prior art relating to the slot width of a shadow mask.